A review is present of the clinical and cardiac morphologic findings in patients (50 previously reported and 3 new patients) with histiocytoid cardiomyopathy. This disorder occurs in infants and small children and is characterized clinically by severe, ofter fatal arrhythmias, and morphologically by focal collections of altered myocytes that are tound- shaped and resemble histiocytes. Evidence is presented to exclude the possibilities that the disorder represents a developmental anomaly of the atrioventricular conduction system, a multifocal tumor of Purkinje cells, a developmental arrest of cardiac myocytes and a diffuse type of mitochondrial cardiomyopathy. Histiocytoid cardiomyopathy is considered to be the result of hamartoma-like aggregations of cardiac myocytes with features similar to those of oncocytes. This syndrome is likely due to prenatal myocardial or systemic (viral?) injury. Surgical excision of histiocytoid cells can result in clinical remission.